This invention relates to a device for measuring distance in the radial direction between walls of a pair of coaxial cylindrical screens such as catalyst holding screens of a radial flow reactor.
A radial flow reactor used in petrochemical industries has a pair of coaxial cylindrical screens erected in a vertical direction. A catalyst layer is formed by filling catalyst in an annular space formed between the inner screen and the outer screen of the pair of cylindrical screens (this annular space will hereinafter be called "annulus" according to the general terminology used in the art) and a desired chemical reaction is performed by causing gas to be treated to pass through the catalyst layer from the outer screen toward a hollow space inside of the inner screen. For producing a uniform product, the distance in the radial direction between the inner wall of the outer screen and the outer wall of the inner screen, i.e., the thickness of the annulus, must be uniform. In a case where a radial flow reactor is installed, it is necessary to measure accurately the distance in the radial direction between the outer and inner screens in many positions both in the circumferential and vertical directions before filling the catalyst in the annulus.
U.S. Pat. No. 5,392,527 discloses a device for automatically performing this measurement. This prior art device has a wheeled cart having two pairs of wheels which are mounted such that they straddle on an inner screen with a certain angle and a pair of spring-loaded plungers including ball-like contact members. This device is lowered by means of a cable in an annulus between inner and outer screens and a pair of supersonic sensors positioned in the cart issue beams oppositely in the radial direction in the annulus to measure the distance in the radial direction between the inner and outer screens.
This prior art measuring device is convenient in that it has obviated a troublesome manual measurement. This measuring device which performs measurement according to the method in which it issues detecting beams of a supersonic sensor or a laser sensor to the outer wall surface of the inner screen and the inner wall of the outer screen and measures time required for retroflex reflection of the beams from the outer wall and inner wall has the following problems:
As inner and outer screens of a radial flow reactor, a wedge-wire screen is generally used. Since the diameter of detecting beam of a supersonic sensor is about 5 mm, in a case where the width of slit of the wedge-wire screen is larger than the diameter of the detecting beam, an accurate measurement is possible when the detecting beam strikes against a flat surface of the wedge-wire but, when the detecting beam enters the slit of the wedge-wire, the detecting beam passes through the slit and strikes against a reinforcing panel or a support rod provided behind the wedge-wire and is reflected therefrom. As a result, an error in the measurement takes place. Further, when the detecting beam does not strike against the flat surface of the wedge-wire but strikes against an inclined portion at the edge of the wedge-wire, though it may not pass through the slit, an accurate retroflex reflection cannot be obtained and, as a result, an error in the measurement takes place.
It is, therefore, an object of the invention to provide a device for measuring distance in the radial direction between walls of coaxial cylindrical screens which are free from errors in the measurement caused by the above described measurement method utilizing a detecting beam.